Marine antifouling paints comprising organic or inorganic biocides admixed with resinous binders such as vinylchloride resin, alkyd resin or the like have been widely used. However, since the antifouling effect is dependent, in any case, upon the amount of biocide dissolved out of the coating surface, and the dissolving rate of said biocide is principally based on diffusion phenomenon due to concentration gradient of said toxicant in the coating, a stable and longer term antifouling effect cannot be expected therewith. Furthermore, after the biocide(s) is (are) dissolved of the coating surface, there remains skeleton structure of water-insoluble resinous component which will cause various problems as increase in frictional resistance of the ship bottom to water, lowered speed, increase in fuel consumption and the like.
Under the circumstances, there has moved into the limelight an antifouling paint comprising an antifouling agent and a hydrolyzable type resinous vehicle, which can give a comparatively tough coating whose resin is, under sailing conditions, gradually hydrolyzed and dissolved in sea water. One of the most attractive known hydrolyzable resins used in such application would be an acrylic resin having, as a component, a tri-organic tin .alpha.,.beta.-unsaturated carboxylate. This is because this high molecular weight organic tin polymer can be hydrolyzed in weak alkaline sea water, liberating the tri-organic tin portion therefrom and resulting in a resin with hydrophilic carboxyl groups. In order to give a stable and tough coating, the resin in the coating should preferably be of a high molecular weight with the least possible hydrophilic groups, whereas to give a sufficient dissolution of the resin in water, the content of said hydrophilic groups should be in excess of a defined level. To cope with these two conflicting requirements, there has been a process of preparing a polymer of tri-organic tin .alpha.,.beta.-unsaturated carboxylate and acrylic vinyl monomer, using a comparatively higher concentration of the former and selecting a member with the least possible hydrophilic groups as the latter. In fact, various polymers of acrylic esters, styrene or the like containing 55 to 65% by weight of tri-organic tin .alpha.,.beta.-unsaturated monocarboxylates have been developed and placed on the market. The acrylic vinyl monomers actually polymerized with said organic tin monomers did include those with hydrophilic groups such as acrylic acid. However, acrylic acid was only a part of the polymerizable monomers used, with an understanding that its presence was undesirable in substance, and even in such case, it has been believed that the amount of tri-organic tin .alpha.,.beta.-unsaturated carboxylate should be as high as possible, i.e. in the case of monocarboxylate, at least 55% by weight, to give an adequate water solubility to the hydrolyzed resin. This type of acrylic resin containing as a component a higher concentration of tri-organic tin .alpha.,.beta.-unsaturated carboxylate is indeed quite useful for the intended object since it is gradually hydrolyzed and dissolved in sea water, liberating a tin compound as an antifouling agent. However, from the standpoint of public health, it is of course desired to decrease the amounts of tin compound liberated from said hydrolysis with all possible means. Also, since tri-organic tin compounds are expensive, the reduction of the amount of said ingredient is welcomed from this point of view. And, in the case of antifouling paint comprising a hydrolyzable resinous vehicle, since the coating is gradually decomposed and dissolved in sea water, the desired antifouling effect would be likely attained, without relying on the substance liberated alone, by the inclusion or admixture of other biocides known per se.
With the abovesaid in mind, the inventors have made an endeavor to obtain a resinous composition for antifouling paint comprising a film-forming acrylic polymer containing, as a polymer constituting unit, a tri-organic tin .alpha.,.beta.-unsaturated carboxylate, the content of said tin carboxylate being cut as much as possible and at least to the level of not more than 55% by weight, and still exhibiting an excellent film-forming property and the same extent of polishing effect as attained with heretofore known antifouling paints of this type, and have finally come to the present invention.